@Neill: You write "Most installations seem to run fine" When installing Lotus, did you have NTFS.ifs running. If so, rem it in config.sys, clean the Lotus Smartsuite installation and reinstall. Then all Lotus Smartsuite applications should work, and Lotus should be able to locate default.mwp.
If this is not the problem, and you have a personalized directory for smartmasters etc., try copying this file to that directory.